The invention relates to a straightedge arranged by means of cables to provide a parallel edge across the drawing table for drafting and graphic art purposes. In the movement of the conventional straightedges along the table, particles of graphite and dust become trapped between the bottom surface of the straightedge and the paper causing the paper to become smudged. An early attempt to avoid smudging is found in U.S. Pat. No. 2,234,467 entitled "Draftsman's Straightedge" wherein a plurality of rollers are arranged on the bottom surface of the straightedge to provide for clearance between the straightedge bottom and the paper. In this arrangement, the rollers themselves traverse across the paper and sometimes trap the graphite and dust particles in the vicinity of the rollers. Although the straightedge does not rest directly on the drawing, the rollers themselves roll on the surface and cause smudging along the path they traverse.
U.S. Pat. No. 1,584,839 entitled "Ruling Edge Device" also describes an earlier attempt to reduce smudging when straightedge devices are moved across a drawing table.
In both of these patents, the smudging is localized to the regions of the straightedge that trap the graphite and dust particles and, in some instances, actually intensify the smudging problem.
Another problem involved with movement of a straightedge across a drawing table is that of holding the straightedge in a restrained movable position on an inclined drafting table as well as a fixed position on a table which is horizontal or inclined. Most straightedges that are supported by means of cables have braking systems which clamp the cable to the straightedge causing the cable to become abraided over periods of continuous use eventually resulting in cable breakage.
When the integral transparent edge of straightedges are used as a guide for sharp instruments while cutting paper, film etc., the edge is frequently nicked thereby rendering the straightedge useless as a precision drawing instrument leading to inconvience and the expense of replacement of the entire straightedge assembly.
The parallel straightedge arrangement of the instant invention accordingly provides a smudge-free straightedge with a unique braking system that does not damage the cable while holding the straightedge fixedly in any position. The instant invention also includes a removable transparent plastic insert for drawing straight continuous parallel lines when inserted while allowing the durable metal nick-free edge of straightedge bar to be used as a guide for sharp cutting instruments when the transparent edge is removed.